How to use Ruby's self keyword

There are several important uses, most of which are basically to disambiguate between instance methods, class methods, and variables.

First, this is the best way to define class methods:

class Foo
  def self.bar
    "class method bar"
  end

  def bar
    "instance method bar"
  end
end

Foo.bar  #returns "class method bar"

foo = Foo.new
foo.bar #returns "instance method bar"

Also, within instance methods self refers to the instance, within class methods it refers to the class, and it can always be used to distinguish from local variables.

class Bar
  def self.foo
    "foo!"
  end

  def baz
    "baz!"
  end

  def self.success
    foo #looks for variable foo, doesn't find one, looks for class method foo, finds it, returns "foo!"
  end

  def self.fail
    baz #looks for variable baz, doesn't find one, looks for class method baz, doesn't find one, raises exception
  end

  def instance_success
    baz #looks for variable baz, doesn't find one, looks for instance method baz, finds it, returns "baz!"
  end

  def instance_fail
    foo #looks for variable foo, doesn't find one, looks for instance method foo, doesn't find one, raises exception
  end

  def local_variable
    baz = "is my favorite method"
    baz #looks for variable baz, finds it, returns "is my favorite method"
  end

  def disambiguate
    baz = " is my favorite method"
    self.baz + baz #looks for instance method baz, finds it, looks for local variable baz, finds it, returns "baz! is my favorite method"
  end
end

So, in the end, you can avoid using self in many cases, but it's often helpful to use it to make sure that you don't inadvertently create naming conflicts later on. Sometimes those can create bugs that are very hard to find. In the end it's often a matter of personal style.

---

As noted in the comments, one more really important thing:

In a class, if you have a method like this:

def bar=(string)
  ...
end

And in another method you call:

def other_method
  bar = "abcd"
end

It isn't going to call your bar= method, it's going to create a local variable bar. So, in this case you use self to tell Ruby not to create a local variable:

def other_method
  self.bar = "abcd"
end

The same thing applies if you want to take an argument with the name of a method:

def example
  ...
end

def other_thing(example)
  self.example(example)
end

If you left off self Ruby would assume you meant the local variable with the same name.

So, in general, self in method names is used to distinguish between class and instance variables, and everywhere else you use it when Ruby needs help distinguishing between method calls and local variables or local variable assignment.

I hope that makes sense.

In most cases self.foo is indeed redundant because you can just write foo for the same effect, but in this case it is not and the self is required.

var_one = method(args) will create a local variable called var_one, it will not call any method or do anything else to self.

self.var_one = method(args) will call the method var_one= on self with the argument method(args).

Another case where the use of self is non-optional would be if you want to pass it as an argument to a method, i.e. some_method(self) - you can't do that without the self keyword.

Here's an example:

def run miles
  self.miles = miles
end

In this case self will help. In most cases self is redundant.